For example, honeycomb hole structures made of ceramic and having a plurality of through-holes of a cross-sectional polygonal shape are conventionally known. Such honeycomb structures are used, for example, in particulate-matter-removing filters such as diesel particulate filters. In a production step of such honeycomb structures, a ceramic raw material powder is formed by extruding to produce a green honeycomb molded body. A part of through-holes in this green honeycomb molded body are closed at the end surface. A honeycomb structure is produced by firing the green honeycomb molded body with closed through-holes. Patent Literature 1 discloses a method for producing such honeycomb filters. In Patent Literature 1, projections in a closing jig having a plurality of quadrangular pyramid-shaped projections are inserted into a part of the through-holes in the green honeycomb molded body to gather together ends of partition walls that partition the through-holes and to connect the ends of these partition walls, thereby closing the through-holes.